Device for evaluating the signal from a viscosity sensor

ABSTRACT

A device is provided for evaluating the signal from a viscosity sensor having an amplifying circuit. The amplification of the amplifying circuit is controlled by a temperature signal from a temperature sensor.

FIELD OF THE INVENTION

[0001] The present invention is directed to a device for evaluating thesignal from a viscosity sensor.

BACKGROUND INFORMATION

[0002] In previously proposed devices, the signal from the viscositysensor is amplified.

SUMMARY OF THE INVENTION

[0003] The device according to the present invention for evaluating thesignal from a viscosity sensor, in contrast to the related art, has theadvantage that partial compensation is made for the strong influence oftemperature on the measurement variable viscosity. It is thus possibleto greatly reduce the wide divergence in the range of values of themeasurement signal caused by the influence of temperature on theviscosity, thereby permitting higher resolution in the evaluation of themeasurement signal.

[0004] The use of an amplifier which an input for the signal from theviscosity sensor and an additional input for a signal from thetemperature sensor is particularly simple. Amplification of theamplifier is controlled by the signal from a temperature sensor. It isadvantageous if the viscosity sensor and the temperature sensor aresituated in immediate proximity to one another in the measurementliquid; this ensures that the measurement values from the viscositysensor and the temperature sensor are based on the same temperature. Thedesign of the viscosity sensor as a microacoustic sensor is particularlysimple.

BRIEF DESCRIPTION OF THE DRAWING

[0005] The FIGURE shows one embodiment of the present invention.

DETAILED DESCRIPTION

[0006] The FIGURE shows a block diagram of the device according to thepresent invention for evaluating the signal from a viscosity sensor. Aviscosity sensor is schematically shown in the FIGURE which produces aviscosity signal V_(S) from the sensor. This signal V_(S) is sent to anamplifying circuit 2, where the signal is amplified and then emitted asoutput signal V. The intensity of this output signal V is a measure ofthe viscosity of a measurement liquid. In addition, a temperature sensor3 is provided which produces a temperature sensor signal T. Signal Tfrom the temperature sensor is sent to an additional input of amplifier2. Amplifier 2 is designed in such a way that the amplification ofamplifier 2 is controlled depending on temperature signal T. Inaddition, temperature signal T is also externally emitted.

[0007] The device described in the FIGURE is suitable for determiningthe viscosity with partial compensation of the effect of temperature onthe viscosity of the measurement medium. In particular, thedetermination of the oil quality in a motor vehicle engine iscontemplated, in which the temperature of the engine oil varies from−20° C. to greater than +120° C. during the measurement operation. Aviscosity sensor is provided in the oil pan for determining theviscosity. The measured viscosity of the engine oil may be used forevaluating the quality, and may serve as a recommendation for an oilchange. Using this measure, an unnecessary oil change may be prevented,or the maintenance interval of a motor vehicle engine may be set up fora correspondingly longer time.

[0008] A microacoustic sensor, for example, is used for the measurementof viscosity. In such microacoustic sensors an oscillating element isinserted into the oil, and the oscillation frequency of the oil or thedamping of this oscillation is a measure of the viscosity. In thefollowing description it is assumed that the oscillation frequency ordamping is converted to an analog signal which represents the viscosity.(The frequency may be converted using a frequency-voltage transformer,for example.)

[0009] In such microacoustic sensors, a sensor signal VS from theviscosity sensor is proportional to density ρ or to viscosity η,according to formula 1:

V _(S)∝(ρη)^(1/2)  (1)

[0010] or alternatively, is proportional to a corresponding reciprocalvalue:

V _(S)∝1/(ρθ)^(1/2)  (2)

[0011] A typical sensor for engine oil has, for example, a sensorcharacteristic for viscosity as expressed by formula 2. In conjunctionwith dependence of the oil viscosity on temperature, this results in anapproximately linear course of the viscosity signal VS from the sensoras a function of the temperature, according to the following formula:

VS≈k 1+k 2 T

[0012] Constants k1 and k2 are constants which depend on the grade ofoil. The dependence of the oil viscosity on temperature may thus becharacterized by two constants. Sensor signal VS is fed into amplifier 2and is amplified with amplification A:

V=A·VS  (4)

[0013] According to the present invention, amplification A is providedwith a temperature dependence so that emitted signal V is influenced bytemperature as little as possible over the intended temperature range,and thus always represents a direct measure of the oil viscosity. Forthe dependence of viscosity on the temperature as represented in formula3, an amplification is provided in the following form:

A=1/(c 1+c 2 T)

[0014] Parameters C1 and C2 are again constants. Naturally, theconstants are most favorably selected such that K1=C1 and K2=C2. Thismakes it possible for output signal V to be a fullytemperature-dependent measure of the oil viscosity which ischaracterized by constants K1 and K2. Usually, constants C1 and C2 maybe selected so that the parameters are very similar for the numerousengine oils, so that the temperature dependence of the viscositymeasurement of engine oils may be significantly decreased. In this way,it is possible to prevent a wide divergence in the value range of Vcaused by the strong dependence of the oil viscosity on temperature.Consequently, subsequent digital processing results in significantlyhigher resolution in the viscosity measurement.

What is claimed is:
 1. A device for evaluating the signal from aviscosity sensor (1) having an amplifying circuit (2) for the signal(VS) from the viscosity sensor, wherein the amplifying circuit (2) has apredetermined temperature dependence for the amplification (A) of thesignal (VS) from the viscosity sensor.
 2. The device as recited in claim1, wherein the amplifying circuit (2) has an input for the signal(V_(S)) from the viscosity sensor and an additional input for a signal(T) from a temperature sensor (3), and the amplification (A) of theamplifying circuit (2) is controlled by the signal (T) from thetemperature sensor (3).
 3. The device as recited in one of the precedingclaims, wherein the viscosity sensor (1) and the temperature sensor (2)are situated in immediate proximity to one another in a measurementliquid.
 4. The device as recited in one of the preceding claims, whereinthe signal (V_(S)) from the viscosity sensor (1) is dependent on thetemperature (T) and the constants (K₁ and K₂) according to the followingformula: VS≈k ₁ +k ₂ T.
 5. The device as recited in one of the precedingclaims, wherein an amplification (A) of the amplifying circuit (2) isdependent on the temperature (T) and the constants (C₁ and C₂) accordingto the following formula: A≈1/(c ₁ +c ₂ T).